How can a bank like Lehman go down so fast

The Reason behind faliure of Lehman Brothers:-
It is overleveraged i.e loan and investment books are much bigger then the capital. Loans were of high risk nature.
.Lehman was buying loan from banks and then package them to sell as bond against the loan. suppose earning on the loan is 6% and bonds are sold at 4% then spread is the earning for investment bank. By selling these bonds it raises money and frees capital.
But when home buyers started defaulting these bonds lost there value. Suppose Lehman faces a redemption and has to repay another bank it has borrowed from.If it sells the mortgage-backed bonds, whose prices have fallen, it will not raise as much as was earlier expected.So, it sells some of the other good assets or bonds which may have nothing to do with mortgages.But since the bank starts dumping these assets, prices of these bonds also dip.This is when the crisis spreads from subprime to prime.
Lehman ws also writer of huge CDs ( credit default swaps). There are collateralised debt obligation (CDOs), credit default swaps (CDSs) and all kinds of derivatives. CDOs are asset (or loan)-backed securities, while CDSs are like a guarantee.Say Bank A lends to a corporate but is unwilling to take the full credit risk. So, Bank A enters into a CDS deal with Bank B; under this, Bank B promises to pay Bank A if the corporate defaults. The money that Bank B earns for this is the CDS premium, which is similar to an insurance premium.Now, if markets turn choppy, risks go up and so does the CDS premium. So, Bank B, which is earning a lower premium has to promote a mark-to-market loss against the CDS position. Here bank B is Lehman. When default on housing loan is mounting the Lehman were suffering huge losses. Because they have never thought that housing prices ever come down. strange!!!